1. Field of the Invention
The present invention relates to electrodes for use in physiologic measurements, including measurements for medical diagnostic procedures, and more particularly to an electrode that is stretchable or elastic in at least one direction to provide reliable low impedance electrical contact with the skin. As used herein in connection with the invention, the terms “elastic,” “stretchable” and “elastically stretched” refer to the property of a material or element in which a restoring force is created along an axis of the material or element when a compressive force, or a tensile force less than the elastic limit of the material or element, is applied to it along that axis.
The stretchable electrode of the present invention is especially adapted for use in forming reliable, substantially continuous electrical contact with an annular or circumferential region of the skin about a limb, extremity or other portion (neck, chest, penis, etc.) of a human body, e.g., when making peripheral blood flow measurements as described in the aforementioned related applications. The invention also relates to physiologic measuring methods and in particular to a method of using a stretchable electrode for forming reliable, substantially continuous electrical contact with an annular or circumferential region of the skin about a limb or extremity of a human body, for example, in medical diagnostic procedures, such as peripheral impedance plethysmography.
2. Description of the Prior Art
The need for making reliable electrical contact with the skin of the human body during physiologic measurements, including medical diagnostic procedures is self-evident. Direct contact of conventional metal electrodes with the skin generally provides an unreliable electrical contact, especially circumferential electrical contact with the skin about a limb of the body. Heretofore, electrical contact with the skin has typically been accomplished using disposable electrodes that are coated with or embedded in a conductive paste or adhesive layer. Adhesive-coated electrodes are sometimes uncomfortable for the patient, especially when they are removed from the skin. Paste-coated electrodes often do not make the necessary low impedance electrical contact with the skin that is needed for some physiologic measurements and require cleanup after use.
For certain diagnostic procedures, such as monitoring heart rate while exercising on a treadmill, i.e., a “stress test,” the use of disposable, adhesive- or paste-coated electrodes is sometimes neither practical nor economically feasible. In such circumstances, it would be desirable to have a reusable electrode that makes reliable low impedance electrical contact with the skin without the need for a conductive adhesive or paste. In still other diagnostic procedures, adhesive- or paste-coated, disposable electrodes often do not provide sufficient electrical contact with the skin that is necessary for achieving accurate results.
To make high quality impedance plethysmographic measurements of a body segment, a circumferential quadripolar electrode system can be employed which uses two outer current source electrodes paired with two inner voltage measurement electrodes. A typical quadripolar electrode system is shown in FIG. 1 labeled “Prior Art.” A constant alternating current is applied to the outer electrodes A, B. The voltage is then measured between the inner electrodes C, D. The impedance of the limb segment L between the inner electrodes can then be calculated.
The current distribution through the limb segment L must be as uniform as possible. If there are gaps in the contact of the circumferential current source electrode with the skin, there will be portions of the limb segment through which there is little or no current and the volume of the limb segment will be underestimated. This will result in inaccurate measurements. To make reliable, continuous, circumferential contact with the limb segment, adhesive electrodes using a conductive hydrogel have been conventionally used. Such electrodes must be disposable for hygienic reasons. However, there are applications in which reusable electrodes are preferred, in particular, when frequent spot measurements are made, such as for vascular screening in a physician's office. Disposing of multiple quadripolar electrodes for these tests would be prohibitively expensive.
With respect to a quadripolar electrode system, from a practical standpoint, it is not desirable for the user to have to place four separate electrodes on each limb to create the quadripolar arrangement but, rather, to place each current-voltage pair together in a single array. Because the limbs of the body are not truly cylindrical, but are shaped more in the form of truncated cones, the circumferential lengths of the electrodes must differ to account for the different circumferences of the limb along its length.
U.S. Pat. No. 3,340,867 to Kubicek et al. discloses an impedance plethysmograph which employs four band electrodes positioned to encircle the neck and thorax. The electrodes comprise a pair of excitation electrodes formed from tinned braided copper wires and a pair of measuring electrodes also formed from tinned braided copper wires. According to the patentees, the braid is stretched to a width of about one centimeter and coated on its inner side with an electrode paste to provide low impedance skin contact. Because the braided electrode is stretched out before it is applied to the skin of the neck and thorax, it has no restoring force that aids in maintaining reliable low impedance contact with the skin, i.e., it is not elastic. Consequently, an electrode paste is required to provide the necessary low impedance skin contact.
U.S. Pat. No. 4,452,252 to Sackner discloses an apparatus and method for monitoring cardiac parameters using an extensible conductor adapted to be looped about the neck or other body portion. Changes in the inductance of the loop provide cardiac and pulmonary information. The conductive loop is made extensible by forming an insulated conductor into undulating, loops supported on an elastic tube. The conductive loop does not, however, make electrical contact with the skin of the subject, but is insulated from the skin.
U.S. Pat. No. 4,308,872 to Watson et al. discloses a method and apparatus similar to that of U.S. Pat. No. 4,452,252 to Sackner for monitoring respiration volumes. The apparatus of Watson et al. also uses an extensible conductor attached to the fabric of a tubular stretch bandage disposed on the body of a patient and measures the inductance of the conductor as the patient breathes. The extensible conductor is formed of insulated wire and, like that of Sackner, does not make conductive electrical contact with the skin.
The foregoing and other prior art electrodes used in physiologic measurements have not provided a reusable electrode that is characterized by a reliable low impedance electrical contact with the skin. It would be desirable, therefore, to provide a reusable electrode especially useful for physiologic measurements that can be easily applied directly to the skin without conductive adhesives or pastes and that makes substantially continuous, low impedance electrical contact with the skin of a subject or patient.
It would also be desirable to provide such an electrode that is readily sterilizable by conventional sterilization techniques, e.g., wiping or spraying with sterile alcohol.